The present invention relates generally to field-type electron emitters, and, more particularly, to a system for limiting the effects of arcing in field-type electron emitter arrays, focusing an electron beam generated by the emitter, and controlling individual emitters in an emitter array. A field emitter unit includes a protection and focusing scheme that functions to minimize degradation of the electron beam and allow for focusing of the electron beam into a desired spot size. A control system is provided that allows for individual control of field emitter units in an array with a minimum amount of control channels.
Electron emissions in field-type electron emitter arrays are produced according to the Fowler-Nordheim theory relating the field emission current density of a clean metal surface to the electric field at the surface. Most field-type electron emitter arrays generally include an array of many field emitter devices. Emitter arrays can be micro- or nano-fabricated to contain tens of thousands of emitter devices on a single chip. Each emitter device, when properly driven, can emit a beam or current of electrons from the tip portion of the emitter device. Field emitter arrays have many applications, one of which is in field emitter displays, which can be implemented as a flat panel display. In addition, field emitter arrays may have applications as electron sources in microwave tubes, x-ray tubes, and other microelectronic devices.
The electron-emitting field emitter devices themselves may take a number of forms, such as a “Spindt”-type emitter. In operation, a control voltage is applied across a gating electrode and substrate to create a strong electric field and extract electrons from an emitter element placed on the substrate. Typically, the gate layer is common to all emitter devices of an emitter array and supplies the same control or emission voltage to the entire array. In some Spindt emitters, the control voltage may be about 100V. Other types of emitters may include refractory metal, carbide, diamond, or silicon tips or cones, silicon/carbon nanotubes, metallic nanowires, or carbon nanotubes.
At present, field emitter arrays are not known to be robust enough for use in several potential commercial applications, such as for use in x-ray tubes. Many existing emitter array designs are susceptible to operational failures and structural wear from electrical arcing. Arcing may be more likely to occur in the poor vacuum environment which exists in many x-ray tubes. Most commonly, an overvoltage applied to the gate layer of the emitter device may cause an arc to form between the gate layer and the emitter element, permitting current to flow in a short circuit from the gate layer through the emitter element to the substrate. Another type of arcing is known as insulator breakdown, in which an overvoltage applied to the gate layer can cause a breakdown of an insulating layer positioned between the gate layer and the substrate, which allows current to punch through and create a short circuit between the gate layer and substrate. The arc can also pass over the surface of the insulating layer resulting in what is known as a “flash over.”
When one emitter of an emitter array experiences arcing in either form, or “breaks down,” the insulating layer will no longer be able to support a voltage or electrical bias sufficient for electron emission to continue at the other emitters of the array. In addition, high temperatures produced by the short circuit current can cause wear or damage to the emitter as well as neighboring emitters. Thus, an arc at one emitter can affect the operation of the entire emitter array. It would therefore be desirable to have a system and method which protect an emitter array from the effects of arcing.
When used as an electron source in an x-ray tube application, field emitter arrays create additional challenges beyond those associated with breakdown. For example, certain mechanisms employed for lower voltage requirements in extracting an electron beam from the cathode, such as a grid structure, can increase the degradation of the electron beam quality. Increased beam emittance prevents the electron beam from focusing onto a small, useable focal spot on the anode. As such, the issue of beam quality degradation remains a problem in current field emitter designs.
Another issue with present designs of field emitter arrays is that each of the emitters in the array is addressed in turn via an associated bias or activation line and at appropriate time intervals. Due to the large number of emitter elements in a typical array, there can exist an equally large quantity of associated activation lines and connections. The large number of activation lines need to pass through the vacuum chamber of the x-ray tube to supply the emitter elements, thus there necessitates a large number of vacuum feedthroughs. There is an unavoidable leak rate associated with any feedthrough device, which can lead to gas pressure levels in the tube that can inhibit performance of the emitter elements and their ability to generate electrons.
Thus, a need exists for a system that protects emitter elements in an emitter array from the effects of arcing. It would also be desirable to have a system for controlling the emitter elements that reduces the number of activation lines and feedthrough channels.